static: Handle no-cockpit error

When directly logging into a remote host without cockpit installed, the
login page showed an unhelpful and untranslatable fatal error message:

> Authentication failed: no-cockpit: [Errno 2] No such file or directory

This also left the user with nothing actionable other than reloading the
page.

Handle the "no-cockpit" problem code and show a proper error message.
Use the exact same one as in the shell's host switcher, to avoid
introducing another similar translatable string. This also goes back to
the login form, to either choose another host, or reattempt the login
after installing cockpit on the target.

pkg/static/login.js

@@ -1036,6 +1036,8 @@ function debug(...args) {
                 }
             } else if (xhr.status == 403) {
                 login_failure(_(decodeURIComponent(xhr.statusText)) || _("Permission denied"));
+            } else if (xhr.status == 500 && xhr.statusText.indexOf("no-cockpit") > -1) {
+                login_failure(format(_("A compatible version of Cockpit is not installed on $0."), login_machine || "localhost"));
             } else if (xhr.statusText) {
                 fatal(decodeURIComponent(xhr.statusText));
             } else {

test/verify/check-loopback

@@ -44,17 +44,14 @@ class TestLoopback(testlib.MachineCase):
         b.set_val('#login-user-input', "admin")
         b.set_val('#login-password-input', "foobar")
         b.click('#login-button')
-        b.wait_visible('#login-fatal')
-        self.assertIn("no-cockpit", b.text('#login-fatal'))
+        b.wait_text("#login-error-message", "A compatible version of Cockpit is not installed on localhost.")
 
         m.disconnect()
         self.restore_dir("/etc/ssh", restart_unit=self.sshd_service)
         m.execute("sed -i 's/.*PasswordAuthentication.*/PasswordAuthentication no/' /etc/ssh/sshd_config $(ls /etc/ssh/sshd_config.d/* 2>/dev/null || true)")
         m.execute(self.restart_sshd)
         m.wait_execute()
 
-        b.reload()
-        b.wait_visible("#login")
         b.set_val('#login-user-input', "admin")
         b.set_val('#login-password-input', "foobar")
         b.click('#login-button')